Rename the empty mutation type and hide __ fields from introspection

This makes GraphiQL stop emitting deprecation warnings

src/schema/model.rs

@@ -117,7 +117,7 @@ impl<'a> SchemaType<'a> {
         SchemaType {
             types: registry.types,
             query_type_name: query_type_name,
-            mutation_type_name: if &mutation_type_name != "__EmptyMutation" { Some(mutation_type_name) } else { None },
+            mutation_type_name: if &mutation_type_name != "_EmptyMutation" { Some(mutation_type_name) } else { None },
             directives: directives,
         }
     }

src/schema/schema.rs

@@ -34,6 +34,9 @@ impl<'a, CtxT, QueryT, MutationT> GraphQLType for RootNode<'a, QueryT, MutationT
 graphql_object!(<'a> SchemaType<'a>: SchemaType<'a> as "__Schema" |&self| {
     field types() -> Vec<TypeType> {
         self.type_list()
+            .into_iter()
+            .filter(|t| t.to_concrete().map(|t| t.name() != Some("_EmptyMutation")).unwrap_or(false))
+            .collect()
     }
 
     field query_type() -> TypeType {
@@ -84,6 +87,7 @@ graphql_object!(<'a> TypeType<'a>: SchemaType<'a> as "__Type" |&self| {
                 Some(fields
                     .iter()
                     .filter(|f| include_deprecated || f.deprecation_reason.is_none())
+                    .filter(|f| !f.name.starts_with("__"))
                     .collect()),
             _ => None,
         }

src/types/scalars.rs

@@ -166,7 +166,7 @@ impl<T> GraphQLType for EmptyMutation<T> {
     type Context = T;
 
     fn name() -> Option<&'static str> {
-        Some("__EmptyMutation")
+        Some("_EmptyMutation")
     }
 
     fn meta<'r>(registry: &mut Registry<'r>) -> MetaType<'r> {